Definition of fitnah: 

1 – The word fitnah from a linguistic point of view: 

Al-Azhari said: “The Arabic word fitnah includes meanings of testing and trial. The root is taken from the phrase fatantu al-fiddah wa’l-dhahab (I assayed (tested the quality of) the silver and gold), meaning I melted the metals to separate the bad from the good. Similarly, Allaah says in the Qur’aan (interpretation of the meaning): ‘(It will be) a Day when they will be tried [yuftanoona] (punished, i.e. burnt) over the Fire!’ [al-Dhaariyaat 51:13], meaning, burning them with fire.” (Tahdheeb al-Lughah, 14/196). 

Ibn Faaris said: “Fa-ta-na is a sound root which indicates testing or trial.” (Maqaayees al-Lughah, 4/472). This is the basic meaning of the word fitnah in Arabic. 

Ibn al-Atheer said: “Fitnah: trial or test… The word is often used to describe tests in which something disliked is eliminated. Later it was also often used in the sense of sin, kufr (disbelief), fighting, burning, removing and diverting.” (al-Nihaayah, 3/410. Ibn Hajar said something similar in al-Fath, 13/3). 

Ibn al-A’raabi summed up the meanings of fitnah when he said: “Fitnah means testing, fitnah means trial, fitnah means wealth, fitnah means children, fitnah means kufr, fitnah means differences of opinion among people, fitnah means burning with fire.” (Lisaan al-‘Arab by Ibn Manzoor). 

2 – Meanings of the word fitnah in the Qur’aan and Sunnah: 

1-Testing and trial, as in the aayah (interpretation of the meaning):

“Do people think that they will be left alone because they say: ‘We believe, and will not be tested [la yuftanoon]” [al-‘Ankaboot 29:2]

i.e., that they will not be subjected to trial, as Ibn Jareer said. 

2-Blocking the way and turning people away, as in the aayah (interpretation of the meaning):

“but beware of them lest they turn you [yaftinooka] (O Muhammad) far away from some of that which Allaah has sent down to you” [al-Maa’idah 5:49]

Al-Qurtubi said: this means blocking your way and turning you away.

3-Persecution, as in the aayah (interpretation of the meaning):

“Then, verily, your Lord for those who emigrated after they had been put to trials [futinoo] and thereafter strove hard and fought (for the Cause of Allaah) and were patient, verily, your Lord afterward is, Oft‑Forgiving, Most Merciful” [al-Nahl 16:110]

Put to trial means persecuted. 

4-Shirk and kufr, as in the aayah (interpretation of the meaning):

“And fight them until there is no more Fitnah (disbelief and worshipping of others along with Allaah)” [al-Baqarah 2:193]

Ibn Katheer said: this means shirk (worshipping others besides Allaah). 

5-Falling into sin and hypocrisy, as in the aayah (interpretation of the meaning):

“(The hypocrites) will call the believers: “Were we not with you?” The believers will reply: “Yes! But you led yourselves into temptations [fatantum anfusakum], you looked forward for our destruction; you doubted (in Faith) and you were deceived by false desires” [al-Hadeed 57:14]

Al-Baghawi said: i.e., you made yourselves fall into hypocrisy and you destroyed yourselves by means of sin and whims and desires. 

6-Confusing truth with falsehood, as in the aayah (interpretation of the meaning):

“And those who disbelieve are allies of one another, (and) if you (Muslims of the whole world collectively) do not do so [i.e. become allies, as one united block under one Khaleefah (a chief Muslim ruler for the whole Muslim world) to make victorious Allaah’s religion of Islamic Monotheism], there will be Fitnah (wars, battles, polytheism) and oppression on the earth, and a great mischief and corruption (appearance of polytheism)”[al-Anfaal 6:73]

What this means is that “unless believers are taken as close friends instead of disbelievers, even if they are relatives, there will be fitnah on earth, i.e., truth will be confused with falsehood.” This is how it is explained in Jaami’ al-Bayaan by Ibn Jareer.

7-Misguidance, as in the aayah (interpretation of the meaning):

“And whomsoever Allaah wants to put in Al‑Fitnah (error, because of his rejecting of Faith), you can do nothing for him against Allaah” [al-Maa'idah 5:41]

The meaning of fitnah here is misguidance. Al-Bahr al-Muheet by Abu Hayaan, 4/262. 

8-Killing and taking prisoners, as in the aayah (interpretation of the meaning):

“if you fear that the disbelievers may put you in trial [yaftinakum] (attack you)”

[al-Nisa’ 4:101]

This refers to the kaafirs attacking the Muslims whilst they are praying and prostrating, in order to kill them or take them prisoner, as stated by Ibn Jareer. 

9-Difference among people and lack of agreement, as in the aayah (interpretation of the meaning):

“and they would have hurried about in your midst (spreading corruption) and sowing sedition among you [yabghoonakum al-fitnah]” [al-Tawbah 9:47]

i.e., they would have stirred up differences amongst you, as it says in al-Kashshaaf, 2/277. 

10-Insanity, as in the aayah (interpretation of the meaning):

“Which of you is afflicted with madness (maftoon)” [al-Qalam 68:6] Here it means madness.

11-Burning with fire, as in the aayah (interpretation of the meaning):

“Verily, those who put into trial [fatanoo] the believing men and believing women (by torturing them and burning them)”[al-Burooj 85:10]

Ibn Hajar said: the meaning may be understood from the context. (al-Fath 11/176) 

Note: 

Ibn al-Qayyim (may Allaah have mercy on him) said: With regard to the “fitnah” which Allaah attributes to Himself or which His Messenger attributes to Him, as in the aayah (interpretation of the meaning), ‘Thus We have tried [fatannaa] some of them with others’ [al-An’aam 6:53] and the words of Moosa, ‘It is only Your trial [fitnatuka] by which You lead astray whom You will’ [al-A’raaf 7:155 – interpretation of the meaning], that carries a different meaning, which is a test or trial from Allaah to His slaves by means of good things and bad, blessings and calamities. This is one thing; the fitnah of the mushrikeen is another thing; the fitnah or trial of the believer by means of his wealth, children and neighbour is another thing; the fitnah (tribulation, differences) that happen amongst the Muslims, like the fitnah that happened between the supporters of ‘Ali and Mu’awiyah, and between him and the people of the camel, and between Muslims when they fight one another or boycott one another, is another thing. (Zaad al-Ma’aad, vol. 3, p. 170).